Bluetooth: RFCOMM: Fix session timer during acl disc

Session idle timer should not be started in dlc destroy if
session is already disconnected (which can happen in scenarios
like acl disconnection where in dlc is detached from session and
destroy will be called from tx thread) otherwise timer function will
get invoked for a disconnected session.

This patch moves starting of idle timer from dlc destroy to
the point dlc disconnect is recieved from remote since idle
timer is to handle only that scenario.

Change-Id: I94501515a182ff0e2348ba4e2df63100f2f52209
Signed-off-by: Jaganath Kanakkassery <jaganathx.kanakkassery@intel.com>